The invention relates to processes for determining the size distribution of the particles contained in an aerosol, especially the particles of a pharmaceutical formulation.
The invention also relates to an apparatus for carrying out such a process.
Within the scope of the invention the term “pharmaceutical substance” refers to the active ingredient of a medicament which is usually also known as a drug or active substance.
The term “pharmaceutical formulation” is to be interpreted broadly, to cover formulations in the form of solutions, suspensions and powders, in particular. In a solution formulation the pharmaceutical substance is dissolved in a solvent, whereas in a suspension or powder formulation the pharmaceutical substance is present in solid form. Whereas in a suspension formulation it is mixed with a suspension agent and the pharmaceutical substance is contained in this suspension agent in the form of suspended particles, a powder formulation does not have any solvent or suspension agent in this sense but is present to some extent in pure form, as a pure powder.
A solution formulation is prepared and metered using an atomiser or nebuliser, preferably a nebuliser, in which a quantity of less than 100 ml, preferably less than 50 ml, preferably less than 20 ml of the formulation is prepared.
An apparatus of this kind for propellant-free nebulising of a metered amount of the above-mentioned pharmaceutical formulations is described in detail, for example, in International Patent Application WO 91/14468 “Atomizing Device and Method” and also in WO 97/12687, FIGS. 6a and 6b. In a nebuliser of this kind a pharmaceutical formulation is converted into an aerosol by the use of high pressure, up to 500 bar, the particles introduced having a diameter of less than 100 μm, preferably less than 20 μm.
Apart from this device, other inhalers known from the prior art may also be used in the process according to the invention, such as the MDI (metered dose inhaler) or powder inhalers such as the one known by the trademark HandiHaler®, for example.
In nebulisers of this kind the formulations are stored in a reservoir and for this reason the formulations used must be sufficiently stable when stored.
It is essential in the pharmaceutical industry to measure the particle size distributions of aerosols in order to assess the characteristics of deposition in the lungs and bronchial region, as will be shown hereinafter.
In a number of applications, particularly in the case of diseases of the lungs and bronchial region, the pharmaceutical substance is provided in the form of an inhalable medicament. The pharmaceutical formulation is atomised to form an aerosol. The aerosol thus produced can then be transported in a carrier medium, e.g. air.
For example, when an asthma spray is used, a pharmaceutical formulation stored in an atomiser is finely atomised through a nozzle, by brief actuation, and introduced into the ambient air breathed in by the patient, this ambient air acting as the carrier medium. The air enriched with the pharmaceutical formulation forms an aerosol, which is inhaled.
Inhalable preparations demand a certain form for the medicament. As a rule, micronised pharmaceuticals or active substances in solid form are used. However, in theory, the drug may be present in liquid or solid form, e.g. as a powder, while solid particles do not dissolve in the solvent in the traditional sense or are present in pure form.
To ensure that the pharmaceutical substance is capable of being inhaled, stringent demands are made of the particle size, particle size distribution, morphology, stability and flow characteristics.
As a rule, not all the inhaled dose of the pharmaceutical substance reaches the lungs but only part of this dose. The amount of the composition which actually enters the lungs is critically influenced by the particle size. For this reason, particles with a diameter of less than 20 μm, preferably less than 5 μm and greater than 0.3 μm are preferred.
The diameter of the particle should fall within the range specified and should additionally have the narrowest possible size distribution. Larger particles are deposited too early, in the upper respiratory tract, when breathed in, whereas smaller particles are not deposited in the lungs and are breathed out again.
For example, when an asthma spray is used, a pharmaceutical formulation stored in an atomiser is finely atomised through a nozzle, by brief actuation, and introduced into the ambient air breathed in by the patient, this ambient air acting as the carrier medium. The air enriched with the pharmaceutical formulation forms an aerosol, which is inhaled.
To ensure that the pharmaceutical substance is capable of being inhaled, stringent demands are made of the particle size, particle size distribution, morphology, stability and flow characteristics.
As a rule, not all the inhaled dose of the pharmaceutical substance reaches the lungs but only part of this dose. The amount of the composition which actually enters the lungs is critically influenced by the particle size. For this reason, particles with a diameter of less than 20 μm, preferably less than 5 μm and greater than 0.3 μm are preferred.
The diameter of the particle should fall within the range specified and should additionally have the narrowest possible size distribution. Larger particles are deposited too early, in the upper respiratory tract, when breathed in, whereas smaller particles are not deposited in the lungs and are breathed out again.
By the particle diameter within the scope of the present invention is meant the aerodynamic particle diameter, which is defined as the equivalent diameter of a sphere with a density of 1 g/cm3 which has the same sedimentation speed in air as the particle under investigation.
Against this background it is easily understandable that the pharmaceutical industry has a need for a process which can be used to determine the particle size distribution of aerosols.
However, the legislators, and particularly the health authorities, also demand accurate knowledge of the dose that is actually administered, i.e. the proportion of the total dose inhaled which is deposited in the lungs and bronchial region.
Moreover, apart from the absolute quantity administered, the size distribution affects the bioavailability of the pharmaceutical substance in that, although the absolute amounts are the same, a large number of small particles have a different bioavailability from a small number of large particles.
According to the prior art, three conventional methods are used to determine the particle size distribution.
A first, widely used method of determining particle size distribution is the so-called impaction method using the Andersen cascade impactor. The cascade impactor is a standardised apparatus for carrying out a standardised measuring process, the so-called impaction method; both the process and the apparatus are described in detail in drugs manuals (cf. also European Pharmacopoeia, 3rd Edition, Supplement 2001, 2.9.18 Preparation for inhalation).
FIG. 1 shows the Andersen cascade impactor in diagrammatic side view and partially in section (loc. cit. page 122). The cascade impactor (1) is acted upon by the aerosol which is under investigation through the inlet opening (3) of a right-angled inlet tube (2).
The inlet (2) is a standardised component (loc. cit. page 120) which is also known as a USP throat and simulates the oropharyngeal-cervical cavity in humans. To illustrate the USP throat, FIG. 2.9.17-7 (induction port) is reproduced in FIGS. 2a to 2d. 
FIG. 2d shows the USP throat in perspective view, while FIGS. 2a to 2c serve to illustrate the dimensions envisaged. FIGS. 2a to 2d are intended to give an overall impression of the USP throat and show that it is a component with an extremely detailed specification, leaving no leeway for the manufacturer or user.
As with the pharmaceutical formulation administered to the patient with the ambient air breathed in, with which it forms an aerosol, is passed through the oral and pharyngeal cavities into the windpipe and from there is passed into the lungs to the bronchi, in the Andersen cascade impactor (1) as well the aerosol is conveyed along a curved flow path through the non-linear USP throat (2) to the actual sample collector (5).
In accordance with human anatomy, the aerosol flow through the entry opening is conveyed into a first section (2a) of the USP throat (2) and then into a second section (2b) which is connected to the first section (2a) and arranged substantially perpendicular thereto.
The particles of the aerosol are subjected to radially outwardly directed centrifugal forces on account of the nonlinear direction of flow and the resulting curved flow path. If the mass of the aerosol particles exceeds a certain size, these particles can no longer follow the deflected flow but are deposited on the walls of the USP throat (2).
FIG. 1 shows the flight path (12) of a particle which cannot follow the direction of flow and hits or is deposited on the inner wall of the second section (2b) of the USP throat (2).
This is in principle the first stage of the Andersen cascade impactor which simulates the deflection of the aerosol breathed in by the patient in the pharyngeal cavity and the resulting deposition of pharmaceutical formulation in the pharyngeal cavity.
The USP throat (2) is connected to the actual sample collector (5) via a connecting member (4), which is also standardised (loc. cit., page 123). The aerosol flow expands in the connecting member (4) and is guided towards the first stage or cascade (61) of the cascade impactor (1)
The cascade impactor (1) is a substantially cylindrical container of modular construction through which the aerosol fed in travels from top to bottom, passing through a number of stages, the so-called cascades, while the aerosol particles contained in the carrier medium are deposited in a sequence from coarse to fine or from heavy to light.
Each stage or cascade (61, 62, 63, 64, 65, 66, 67, 68, 69) comprises a plurality of impactor nozzles (71, 72, 73, 74, 75, 76, 78). An impactor nozzle (7) of this kind is shown diagrammatically in side view and in section in FIG. 3.
The aerosol which acts on the nozzle (7) is deliberately accelerated in the inlet aperture (8) of the nozzle (7) by a defined constriction of the cross section of the nozzle entrance and then deflected by means of an impactor plate (11). As in the deflection of flow in the USP throat (2), here too the curved path of movement and the centrifugal forces acting on the particles as a result cause particles of a certain mass to be deposited.
FIG. 3 shows the flow lines (101, 102) of the aerosol flow, which the lighter particles essentially follow without colliding with the impactor plate (11). FIG. 3 also shows the flight path (12) of a particle striking the impactor plate (11) because of its excessively great mass.
The nozzle (7) acts to some extent as a filter for filtering out particles exceeding a given mass from the aerosol flow and depositing them on the impactor plate (11). Because of the fact that it is a standardised apparatus and a standardised process, accurate information is available as to the conditions in the region of the nozzle. For each cascade the precise mass mab of the particles deposited on the impactor plate (11) here is known.
After passing through the first stage or cascade (61) and after the first depositing of heavy particles, the aerosol passes through eight more cascades (62 to 69) as shown in FIG. 1, while the geometry of the impactor nozzles (7) varies or becomes finer from stage to stage and allows finer and finer, i.e. lighter, particles to be filtered out.
The aerosol particles deposited in a certain stage thus have a specific mass which is within a very narrow window bounded by an upper and lower limit.
As the last stage (not shown in FIG. 1) a filter may be provided which collects all the particles that have not previously been deposited and thus, together with the impactor plates (11), makes it possible to determine the absolute total mass of the pharmaceutical formulation fed into the impactor.
After the aerosol has passed through the impactor, the impactor plates (11) of each cascade are removed and subjected to extensive analysis. The main priority is to determine the particle size distribution. Theoretically, first of all the total mass of pharmaceutical formulation impacted or deposited on each impactor plate could be determined. By knowing the mass mab of the particles deposited in each stage the number of particles deposited in each cascade can be calculated.
In practice however, it has been found that the measurements thus obtained are not reproducible within narrow limits. Tests have shown that some of the pharmaceutical formulation evaporates during the measuring process.
Because of its unsaturated state the carrier medium can absorb additional liquid, and therefore liquid may be, and generally is, given off from the particles to the carrier medium by evaporation.
The evaporation of the particle droplets leads to a change in the particle mass of each individual particle and hence to a reduction in the particle diameter and consequently to a measurement which is falsified by evaporation.
The fact that the effects of evaporation may be significant is demonstrated in FIG. 4, which shows the lifespan of a drop of water depending on the initial droplet diameter for various relative humidities (0%, 50%, 100%) at 20° C. (William C. Hinds “Aerosol Technology—Properties, Behavior, and Measurement of airborne Particles”, page 270, ISBN 0-471-08726-2).
Small particles have a short lifespan and may evaporate during measurement, so that they are no longer taken into consideration within the scope of the particle size distribution.
For the reasons stated it is not sufficient just to determine the total mass of the pharmaceutical formulation deposited on each impactor plate. Moreover, the pharmaceutical formulation of each impactor plate has to be subjected to analysis to determine the concentrations of the contents. Partial evaporation of the solvent or suspension agent causes concentration of the pharmaceutical substance and the other ingredients of the aerosol droplet.
On the basis of the degree of concentration of the pharmaceutical substance conclusions are drawn as to the fraction of the particle which is evaporated. Taking this evaporated fraction into consideration acts as a corrective when determining the particle size distribution and leads to a different total mass for the pharmaceutical formulation deposited on each impactor plate.
The pharmaceutical formulation deposited on the impactor plates (11) may, for example, be analysed for its composition by the HPLC method.
Further tests have shown that in spite of taking account of the evaporation as described above, the results still vary from one measurement to the next and it is desirable to increase the reproducibility still further. Other tests have shown that simply allowing for the mass of the evaporated fraction of the aerosol particles is not enough.
If a liquid pharmaceutical formulation contains acids or bases to adjust the pH, the evaporation that occurs during measurement leads to a raising or lowering of the pH. Consequently, in pH-sensitive active substances, decompositions occur, with the result that the analysis of the concentration of the active substance in the fractions deposited no longer corresponds to the concentration which was actually present in the original aerosol droplet. This procedure thus comes up against its limitations and can only be used to a limited extent as a corrective for the evaporation effect. A pharmaceutical solution or suspension might, for example, contain an acid X as excipient, in addition to the pharmaceutical substance A and water as the solvent. If some of the solvent water is evaporated, the pH is lowered and the particle becomes increasingly acidic, triggering breakdown of the pharmaceutical substance. The same is true if the pharmaceutical formulation contains a base.
The analysis, i.e. the evaluation of the measurements made with the cascade impactor, is extraordinarily time-consuming and labour-intensive. The entire apparatus is taken to pieces in order to gain access to the multiplicity of impactor plates (11). Each impactor plate is weighed and analysed. Thus, as a rule, only a few measurements can be done per day and there is a considerable time span between the actual measuring and the results of the measurements becoming available.
Another process for determining the particle size distribution of an aerosol, which is far less time-consuming and labour-intensive than the impactor method, is the so-called laser diffraction method. Unlike the impactor method the laser diffraction method does not require any complex analysis and therefore makes it possible to work considerably faster and to obtain the results of the measurements much more quickly.
DIN-ISO 13320-1 (First Edition 1999 Nov. 1) describes laser diffraction processes. In them, parallel light is transmitted perpendicular to an aerosol flow using a laser. The particles contained in the aerosol flow obstruct the laser beams, with the result that the light beams are diffracted on the particles. The scattered light emerging at the opposite side of the incident laser beam, which is generated by the diffraction of the laser beams on the particles, produces a circular interference pattern with concentric rings and is fed to a detector, usually a semiconductor detector. The usual methods of evaluating this interference pattern are Mie scattering and the Fraunhofer method.
Whereas the aerosol flow to be investigated is generally fed continuously to the cascade impactor through a USP throat, the particle size distribution of an aerosol is determined by the laser diffraction method according to the prior art on the free-flowing aerosol, i.e. usually on a one-off, conical, inhomogeneous and therefore non-reproducible metered stream.
However, a disadvantage of the laser diffraction method, as with the impactor method, is the fact that some of the aerosol particles are evaporated and thus the measurements are falsified. Although measurement of the aerosol by the laser diffraction method can be carried out much faster than the impactor method, in which the aerosol flow has to travel along a long flow path, the evaporation effect also plays a part in the laser diffraction method.
Precisely in the case of aerosols in which the particles are in the form of drops of liquid there is a danger of at least partial evaporation of the liquid aerosol particles, which means that the effects described are particularly significant in formulations in the form of solutions and suspensions.
The functional correlation shown in FIG. 4 and the need to take it into account were verified experimentally.
Experiments with the cascade impactor at various relative humidities have shown that measurements of the particle size distribution of an aerosol should most sensibly be carried out at high relative humidities, as the evaporation effect crucially influences the measurements if the humidity of the air is too low and finally high humidity levels also correspond to the actual conditions in the human oropharyngeal-cervical cavity. It should be taken into consideration that the processes described are to be used to determine the characteristics of deposition in the lungs and bronchial region, and for this reason every attempt should be made to simulate the conditions prevailing therein, i.e. pressure, temperature and humidity.
A third method of measuring aerosols is the scattered light method. Such a method is described by Dr.-Ing. H. Umhauer in VDI Berichte 232 (1975), pages 101ff. “Ermittlung von PartikelgröBenverteilung in Aerosolströmungen hoher Konzentration nit Hilfe der Streulichtmethode”. 
This method is suitable for measuring very fine particles with a detection limit which may be well within the submicroscopic range. The measuring process is set up as a counting process which detects the particle size and counts the individual particles so that it is possible to make pronouncements as to both the quality and quantity of the particles.
A disadvantage of the scattered light method is that a small measuring volume has to be defined, e.g. with sides 100:m long, because only one particle may ever stop in the measuring volume if clear scattered signals are to be obtained. The apparatus and the calibration thereof are correspondingly complex. In practice the scattered light method has proved to be inferior to the laser diffraction method as its results are less reliable.
Moreover, the scattered light method, like the laser diffraction method, suffers from the evaporation effect with the associated disadvantages.
As in the conventional laser diffraction process, the scattered light method is also carried out on a free flow of aerosol.
To sum up, it can be said of the prior art that the cascade impactor is highly time-consuming and costly because of its complicated analysis, whereas the scattered light method and also the laser diffraction process, while offering comparatively fast processes, suffer from the evaporation effect, like the cascade impactor.
Against this background the problem of the present invention is to provide a process for determining the particle size distribution of an aerosol which counteracts the disadvantages of the prior art described, and which minimises the influence of the evaporation of the aerosol particles and increases the reproducibility of the measurements.
A further aim of the process according to the invention is to measure liquid pharmaceutical aerosols for their particle sizes which contain an active substance which responds to changes in pH by breaking down.
Another secondary aim of the invention is to provide an apparatus for carrying out such a process.